1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use as a band-pass filter or other filter in an RF stage of a portable telephone or electronic apparatus, and more particularly, to a longitudinally coupled resonator type surface acoustic wave device having a plurality of interdigital transducers (IDTs) arranged in the propagation direction of a surface acoustic wave.
2. Description of the Related Art
As band-pass filters used in the RF stage of portable telephones, surface acoustic wave filters have been widely used. The band-pass filters are required to have a low loss, a high attenuation, and a wide band. Accordingly, various attempts to satisfy such requirements have been made in the surface acoustic wave filters.
For example, Japanese Unexamined Patent Application Publication No. 5-267990 discloses a method of increasing the band-width of a longitudinally coupled resonator type surface acoustic wave filter. FIG. 31 shows a longitudinally coupled resonator type surface acoustic wave filter 101 disclosed in the Japanese Unexamined Patent Application Publication No. 5-267990. A distance Z (hereinafter, referred to as an IDT-IDT interval) between the centers of adjacent electrode fingers in neighboring IDT""s shown in FIG. 31 is set at about 0.25 times the wavelength xcexI which is determined by the pitch of the electrode fingers. FIGS. 27 and 28 are graphs for illustration of increasing a band-width in this conventional technique. FIG. 27 shows a relation between generated resonance mode frequencies. FIG. 28 schematically shows the effective current distributions at the respective resonance mode frequencies.
In the method of the above-described conventional technique, a resonance mode (resonance mode indicated by the arrow C) having peaks in the intensity distribution of a surface acoustic wave and presented in the IDT-IDT interval areas, is utilized, in addition to the zero-order mode (resonance mode indicated by the arrow B), and the secondary mode (resonance mode indicated by the arrow A) to form a pass band. Accordingly, ordinarily, the IDT-IDT interval is set at 0.50 xcexI to prevent undesired radiation of a bulk wave. As seen in the above-description of the conventional technique, the band can be widened by setting the interval at 0.25xcexI.
FIGS. 29 and 30 illustrate changes in the resonance mode frequencies indicated by the arrows A to C, obtained when the IDT-IDT interval is varied. The results shown in FIGS. 29 and 30 are obtained and ascertained when the impedance matching conditions are intentionally deviated. It should be pointed out that the results shown in FIGS. 29 and 30 show relative changes in the resonance mode frequencies, not indicating the absolute positions of the accurate resonance mode frequencies.
FIG. 29 shows the shifts of the respective resonance mode frequencies on a basis of the zero-order mode frequency, caused when the IDT-IDT interval is varied, that is, changes in frequency difference between the respective resonance mode frequencies, on a basis of the resonance frequency in the zero-order mode. FIG. 30 shows changes in amplitude level between the respective resonance mode frequencies. As seen in FIGS. 29 and 30, all of the resonance mode frequencies and the amplitude levels are changed when the IDT-IDT interval is varied.
In the above-described conventional technique, the IDT-IDT interval is adjusted so as to increase the pass bandwidth. This will be described with reference to FIG. 32.
FIG. 32 is a schematic plan view illustrating one example of the conventional longitudinally coupled resonator type surface acoustic wave filter.
In this case, a surface acoustic wave filter 200 is produced by forming the respective electrodes made of Al on a 40xc2x15xc2x0 Y-cut X propagation LiTaO3 substrate (not shown). The longitudinally coupled resonator type surface acoustic wave filter 200 has the configuration in which longitudinally coupled resonator type surface acoustic wave filter portions 201 and 202 are two-stage dependently connected. The surface acoustic wave filter portions 201 and 202 are configured in the same manner, and contains first, second, and third IDT""s 205 to 207, and reflectors 208 and 209 provided on both of the sides of the first, second, and third IDT""s 205 to 207. The surface acoustic wave filter portions 201 and 202 are designed in compliance with the following specifications.
electrode finger crossing-width: 43.41 xcexI (xcexI is the wavelength of a surface acoustic wave determined by the electrode finger pitch of an IDT)
the number of the electrode fingers of the IDT (the numbers are 25, 31, and 25 for the IDT""s 205, 206, and 207, respectively)
the wavelength xcexI of an IDT: 4.17 xcexcm
wavelength xcexR for a reflector: 4.28 xcexcm
the number of electrode fingers of a reflector: 100
the intervals D and E between the neighboring IDT""s: 0.32 xcexI
the intervals between the IDT""s and the reflectors: 0.50 xcexR
duty in an IDT: 0.73
duty in a reflector: 0.55
duty in a reflector: 0.55
electrode film-thickness: 0.08 xcexI
It should be noted that in this patent specification, all of the IDT-IDT intervals, the IDTxe2x80x94reflector intervals, and the intervals between adjacent electrode fingers are expressed as the distance between the centers of electrode fingers, respectively. The above-mentioned duty is defined as the ratio of the size in the width direction of an electrode finger along the surface acoustic wave propagation direction, based on the sum of the size in the width direction and the size of the space between the electrode finger and the adjacent electrode finger.
FIGS. 33 and 34 illustrate changes in characteristic of the surface acoustic wave transducer 200 shown in FIG. 32, obtained when the IDT-IDT intervals D and E are decreased by 0.005 xcexI as compared with the above-described design value to increase the bandwidth. The solid lines represents the characteristics obtained when the IDT intervals D and E are decreased, and the broken lines represent the characteristics obtained when the above specifications are applied.
FIG. 33 shows the frequency characteristics, and FIG. 34 shows changes in VSWR.
As seen in FIGS. 33 and 34, the pass bandwidth which corresponds to the range of from the through-level to 4 dB lower than the through-level is increased by about 1 MHz. However, the flatness of the insertion loss within the pass band is deteriorated, and moreover, the VSWR is reduced by about 0.25.
On the other hand, FIGS. 35 and 36 show changes in characteristics of the surface acoustic wave filter 200, obtained when the IDT-IDT intervals D and E are increased by 0.003 xcexI as compared with the abode-described design values, so that the flatness of the insertion loss within the pass band is enhanced, and the VSWR characteristic is improved. The solid lines represent the characteristics when the IDT intervals D and E are increased, and the broken lines show the characteristics obtained when the above-described specifications are applied.
As seen in FIGS. 35 and 36, the flatness of the insertion loss and the VSWR within the pass band can be improved by increasing the IDT-IDT intervals, while the pass bandwidth becomes narrow by about 1 MHz. The reason lies in that when it is attempted to produce a desired characteristic by adjustment of the IDT-IDT intervals, all of the resonance modes are changed, as seen in FIGS. 29 and 30.
That is, since the three resonance modes can not be individually operated, characteristics satisfying all of the pass bandwidth, the flatness of the insertion loss within the pass band, the VSWR, and so forth can not be obtained.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a longitudinally coupled resonator type surface acoustic wave filter which solves the above-described technical defects, and in which the arrangement of the above-described three resonance modes are widely adjustable, and the design flexibility of the pass bandwidth, the flatness of the insertion loss within the pass band, and the VSWR are greatly enhanced.
A preferred embodiment of the present invention provides a longitudinally coupled resonator type surface acoustic wave filter in which the flatness of the insertion loss within the pass band is greatly enhanced, and the VSWR characteristic is greatly improved without the pass bandwidth being decreased.
Another preferred embodiment of the present invention provides a longitudinally coupled resonator type surface acoustic wave filter in which the bandwidth can be increased without the flatness of the insertion loss and the VSWR characteristic within the pass band being deteriorated.
According to an aspect of the present invention, a longitudinally coupled resonator type surface acoustic wave filter is provided which comprises a piezoelectric substrate, and first, second, and third IDT""s provided on the piezoelectric substrate to extend along the surface acoustic wave propagation direction and having plural electrode fingers, respectively, at least one of the first, second, and third IDT""s containing at least one narrow pitch electrode finger portion in which the pitches of a portion of the electrode fingers arranged from the end of the IDT on the neighboring IDT side are shorter than that of the other portion of the electrode fingers in the IDT, the distance between the centers of at least one pair of adjacent electrode fingers differing from 0.25 xcexA+0.25 xcexB in which xcexA represents the wavelength determined by the pitch of one electrode finger, and xcexB represents the wavelength determined by the pitch of the other electrode finger.
In this case, at least one pair of the adjacent electrodes may be not only a pair of adjacent electrode fingers provided in the narrow pitch electrode finger portion and a remaining electrode finger portion neighboring on the narrow pitch electrode finger portion and a pair of the adjacent electrode fingers provided in neighboring IDT""s, respectively, but also a pair of adjacent electrode fingers in an area other than that in which the above-mentioned adjacent electrode fingers are provided.
According to another aspect of the present invention, a longitudinally coupled resonator type surface acoustic wave filter is provided which comprises a piezoelectric substrate, and first, second, and third IDT""s provided on the piezoelectric substrate to extend along the surface acoustic wave propagation direction and having plural electrode fingers, respectively, at least one of the first, second, and third IDT""s including at least one narrow pitch electrode finger portion in which the pitches of a portion of the electrode fingers arranged from the end of the IDT on the neighboring IDT side are shorter than the other portion of the electrode fingers in the IDT, if xcexI2 represents the wavelength determined by the electrode finger pitch of the narrow pitch electrode finger portion, and xcexI1 represents the pitch of the electrode finger portion provided in the IDT including the narrow pitch electrode finger portion and being other than the narrow pitch electrode finger portion, the distance between the centers of the adjacent electrode fingers provided in the narrow pitch electrode finger portion and the electrode finger portion other than the narrow pitch electrode finger portion differing by 0.25 xcexI1+0.25 xcexI2.
According to another aspect of the present invention, a longitudinally coupled resonator type surface acoustic wave filter is provided which includes a piezoelectric substrate, and first, second, and third IDT""s provided on the piezoelectric substrate to extend along the surface acoustic wave propagation direction and having plural electrode fingers, respectively, at least one of the first, second, and third IDT""s including at least one narrow pitch electrode finger portion in which the pitches of a portion of the electrode fingers arranged from the end of the IDT on the neighboring IDT side are shorter than that of the other part of the electrode fingers in the IDT, in the case in which xcexI2 represents the wavelength determined by the electrode finger pitch of the narrow pitch electrode finger portion, and xcexI1 represents the pitch of the electrode finger portion provided in the IDT including the narrow pitch electrode finger portion and being other than the narrow pitch electrode finger portion, the distance between the centers of the adjacent electrode fingers provided in the narrow pitch electrode finger portion and a neighboring electrode finger portion differing by 0.25 xcex1+0.25 xcexI2.
According to still another aspect of the present invention, a longitudinally coupled resonator type surface acoustic wave filter is provided which includes a piezoelectric substrate, and first, second, and third IDT""s provided on the piezoelectric substrate to extend along the surface acoustic wave propagation direction and having plural electrode fingers, respectively, at least one of the first, second, and third IDT""s including at least one narrow pitch electrode finger portion in which the pitches of a portion of the electrode fingers arranged from the end of the IDT on the neighboring IDT side are shorter than that of the other portion of the electrode fingers in the IDT, in the case in which xcexI2 represents the wavelength determined by the electrode finger pitch of the narrow pitch electrode finger portion, and xcexI1 represents the pitch of the electrode finger portion provided in the IDT including the narrow pitch electrode finger portion and being other than the narrow pitch electrode finger portion, the distance between the centers of the adjacent electrode fingers provided in the neighboring IDT""s, respectively, being set to differ by 0.5 xcexI2, when the narrow pitch electrode finger portions are provided adjacently to each other in both of the neighboring IDT""s, and moreover, differing by 0.25 xcexI1+0.25 xcexI2, when the narrow pitch electrode finger portion is provided in only one of the IDT""s, the distance between the centers of the adjacent electrode fingers provided in the narrow pitch electrode finger portion and the electrode finger portion other than the narrow pitch electrode finger portion differing by 0.25 xcexI1+0.25 xcexI2.
Preferably, the deviation amount of the distance between the centers of the adjacent electrode fingers is up to 0.25 xcexI1.
More preferably, the electrode covering ratio of one electrode finger of a pair of the adjacent electrode fingers provided in the neighboring IDT""s is relatively high, wherein the gap between a pair of the adjacent electrode fingers is substantially reduced.
An area between a pair of the adjacent electrode fingers provided in the neighboring IDT""s may be metallized.
According to another aspect of the present invention, a longitudinally coupled resonator type surface acoustic wave filter is provided which includes a piezoelectric substrate, and first, second, and third IDT""s provided on the piezoelectric substrate to extend along the surface acoustic wave propagation direction and having plural electrode fingers, respectively, at least one of the first, second, and third IDT""s including at least one narrow pitch electrode finger portion in which the pitches of a portion of the electrode fingers arranged from the end of the IDT on the neighboring IDT side are shorter than that of the other portion of the electrode fingers in the IDT, an electrode finger portion succeeding the narrow pitch electrode finger portion having a relatively wide electrode finger pitch, an electrode finger portion having a different electrode finger pitch being provided in the narrow pitch electrode finger portion.
Preferably, the distance between the centers of at least one pair of the adjacent electrode fingers provided in the narrow pitch electrode finger portions having different electrode finger pitches, respectively differing by 0.25 xcexA+0.25 xcexB, in which xcexA and xcexB represent the wavelengths determined by the respective electrode finger pitches.
Moreover, preferably, the narrow pitch electrode finger portion is a chirp type electrode finger portion. The chirp type electrode finger portion is configured to have electrode finger pitches arranged in the linearly decreasing order from the center in the surface acoustic wave propagation direction of the IDT toward the outside thereof.
According to the present invention, a longitudinally coupled resonator type surface acoustic wave filter is provided which includes plural longitudinally coupled resonator type surface acoustic wave filters described above and plural-stage dependently connected.
Furthermore, according to the present invention, a communication device is provided which uses a longitudinally coupled resonator type surface acoustic wave filters described above.
Other elements, characteristics, features and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.